This application relates to the art of inspection and sorting of fuel pellets for a nuclear reactor and has particular relationship to the inspection of such pellets for surface flaws and to the evaluation of the flaws on the surfaces of the pellets for the purpose of classifying the pellets. While this application, including its claims, is confined to the processing of fuel pellets for a nuclear reactor, it is to be understood that to the extent that this invention is applicable to the processing of other articles than fuel pellets, such processing is within the scope of this invention.
The pellets whose inspection and classification are the subject of this invention are relatively small cylinders, typically 0.1945.+-.0.002 in diameter, 0.2425.+-.0.020 in length and having a weight of 1.204.+-.0.250 grams and a specific gravity of 10.22. For use in a nuclear reactor a number of these pellets are stacked in a tube of stainless steel or ZIRCALOY alloy to form a fuel rod. The tube is referred to as cladding. For efficient operation of a nuclear reactor and the precluding of hot spots in the reactor, particularly at the start of operation, it is essential that the heat generated in the pellets flow directly and uniformly to the cladding. It is also essential that the fracture or pulverization of the pellets be avoided during loading of the pellets into the cladding tubes. Surface flaws, depending on their shape, tend to cause non-uniform and indirect flow of heat to the cladding and fracture or pulverization of the pellets.
Flaws may be classified as chips and cracks or fissures. A chip is a flaw for which distances between different sets of opposite points of the perimeter are appreciable compared to the dimensions of the pellet and are of reasonably comparable magnitude. A fissure is an elongated flaw whose length is substantially greater than its width. The distinction between a chip and a fissure is subjective. A narrow chip may be regarded as a fissure and a wide fissure as a chip. However, there are surface flaws which are clearly chips and surface flaws which are clearly fissures.
Where a pellet is chipped, the air or space gap between the surfaces of the chip and the cladding is longer than the gap between the surrounding unflawed surface of the pellet and the cladding. Heat which normally flows from the pellet surface to the cladding now encounters a higher thermal resistance path between the surface of the chip and the cladding and seeks an alternate path around the chip. The path which this flow takes around the chip is shared with the heat from the adjacent unflawed portions of the pellet. A hot ring is developed on the pellet surface around the chip and also on the adjacent cladding wall. The average reactor core operating temperature must be decreased to prevent the hot ring which is developed from rising to a temperature which exceeds the safe design temperature for the cladding and can lead to puncture of the cladding. Reduced average core temperature results in reduced thermal efficiency and power generation capacity. Pellets with severe deep or long fissures may fracture and jam while being loaded in the cladding tubes. This may also occur in the case of chips.
To eliminate or at least reduce the number of flawed pellets in an operating reactor and the problems which they raise, the practice in accordance with the teachings of the prior art has been to inspect the pellets manually.
In accordance with common prior art practice, a pellet passed inspection if it met all of the following criteria:
1. The maximum dimension across a chip does not exceed a specified magnitude.
2. The summation of those maximum dimensions across all chips on a pellet which are greater than an inappreciable predetermined magnitude does not exceed another specified magnitude.
3. Fissures do not exceed a specified length.
The inspection task is formidable. A commercial power reactor having an output of 1200 megawatts thermal energy requires about 10 million pellets. To make available this many pellets in a reasonable time demands a large number of inspectors who must inspect the pellets at a high rate. Typically 100 to 200 pellets are deposited on their sides in a grooved tray and examined by the inspector. After mentally noting the degree of fissuring and chipping on each of 100 to 200 pellets, the inspector places a second tray upside down on the first tray and inverts both trays thus turning the pellets over. He now examines the reverse side of the pellets. The inspector then mentally totalizes the damage on each pellet and sorts the pellets into categories according to the observed surface damage. This task is taxing on the memory and judgement capability of the inspector and is nerve-wracking. Another disadvantage of this manual process is that it is applicable only to pellets of weak radiation emitting material such as uranium. It is not applicable to plutonium pellets which cannot safely be handled manually.
For limited use, for example for test reactors, plutonium pellets are inspected in a glove box one-by-one. Two persons are required, one picks up each pellet with tweezers and examines it; the other records the data.
A further disadvantage of the prior art is that the above-listed criteria are not satisfactorially correlated to the pellet performance. For example, a circular chip of maximum passable diameter will produce a more severe hot spot than a chip of the same area which is longer but narrower. If the circular flaw were the only flaw on a pellet, criterion 2 above would pass the pellet containing this chip but would reject a more acceptable pellet having the longer narrower chip.
It is an object of this invention to overcome the drawbacks, disadvantages and difficulties of the prior art and to provide apparatus for reliably inspecting for surface flaws fuel pellets for a nuclear reactor at a high time rate such that the large number of pellets required for a typical commercial reactor can be made available within a reasonable time interval. It is also an object of this invention to provide a method for carrying out such inspection in whose practice the surface quality of each pellet shall be quantitatively evaluated in such manner that it is accurately correlated to pellet performance.